METHODS

Hemizygous males (sZa/-) and homozygous females (sZa/sZa) were progeny of (sZa/-) male and (sZa/sZa) female matings. Heterozygous females (sZa/+) were obtained from (sZa/-) male crosses with normal (C57/BL) females. BALB/c males and females were used as normal controls. Everted gut sacs, approximately 4 cm long, were prepared from unfasted mice as previously described (8). The sacs were filled with 0.4 ml of medium with the following composition, in moles per liter: NaCl, 0.145; CaC12, 0.0001; D-mannose, 0.04; Tris buffer, 0.004, pH 7.4 (Sigma Chemical Co., St. Louis, MO.); and freshly prepared sodium ascorbate, 0.008. Each sac was incubated in 2.5 ml of the same medium in a 25-ml Erlenmeyer flask plus 0.05 pmoles of FeSO4 (final concentration, 2 X 10m5 M FeSO4) and sufficient 5gFeC1 3 to provide adequate counting rates in a well-type scintillation counter. The flasks were incubated under oxygen for 2.5 hr at 37 C in a Dubnoff incubation apparatus.

After incubation, the sacs were removed, drained, and the inside and outside media were analyzed as previously described (8). The average recovery of serosal fluid was 98%(range 78-l 15%) of the initial volume. At least five gut sacs were tested for each variable and the average figures= t 1 SD are given in RESULTS. The net transport of 5gFe was calculated as follows: net serosal transfer equaled the total 5gFe recovered in the serosal medium after incubation; net mucosal uptake was equal to the difference between the total initial 59Fe in the mucosal medium and the total 59Fe recovered in the mucosal medium after incubation. In order to exclude the possibility of isotope exchange with endogenous iron and resultant changes in medium specific activity, inside and outside media from four groups of five gut sacs were pooled in two experiments and chemical iron was determined using the a, cr’-dipyridyl method of Ramsay (16). During the incubation, the initial specific activity of the mucosal medium decreased approximately 15%; but the final specific activities of the serosal and mucosal media were approximately equal, indicating relatively little exchange of isotopic iron for stable tissue iron. The volume and iron concentration of the media were insufficient for the routine determination of chemical iron and only 59Fe data are reported here. The absolute values of net transport in similar groups of animals in separate experiments may vary since the data were not adjusted for different initial mucosal medium specific activities. The methods for the study of 45Ca, proline, and galactose transport previously described (5) were modified for the present experiments as follows: duodenal gut sacs 3.5 cm